1. Field of the Invention
The invention relates to a JBOD subsystem, and more particularly, to a JBOD subsystem having a plurality of physical storage devices for providing storage to a host entity through an emulation controller.
2. Description of the Prior Art
Storage virtualization is a technology that has been used to virtualize physical storage by combining sections of physical storage devices (PSDs) into logical storage entities, herein referred to as logical media units (LMUs), that are made accessible to a host system. In a LMU, individual storage elements (e.g., storage blocks) are uniquely addressable by a logical storage address. One common example of a LMU is the presentation of the physical storage of a HDD to a host over the host IO-device interconnect. In this case, while on the physical level, the HDD is divided up into cylinders, heads and sectors, what is presented to the host is a contiguous set of storage blocks (sectors) addressed by a single logical block address.
An External (sometimes referred to as “Stand-alone”) Storage Virtualization Controller is a Storage Virtualization Controller that connects to the host system via an IO interface and that is capable of supporting connection to devices that reside external to the host system and, otherwise, operates independently of the host. The JBOD emulation controller of the present invention is one example of an external Storage Virtualization Controller.
Traditional JBOD (Just a Bunch Of Drives) subsystems have incorporated Parallel SCSI or Fibre FC-AL physical storage devices (PSDs) because the nature of the interface architecture allows multiple devices to be connected onto a single interconnect. The internal interconnect simply serves as an extension of the interconnect cable that connects the JBOD subsystem to the host.
Please refer to FIG. 1 and FIG. 2. FIG. 1 shows a block diagram of a JBOD subsystem 14 incorporating Parallel SCSI PSDs according to the prior art, while FIG. 2 shows a block diagram of a JBOD subsystem 24 incorporating Fibre FC-AL PSDs according to the prior art. In FIG. 1, a computer system 10 contains a JBOD subsystem 14 having a plurality of Parallel SCSI PSDs 16 for providing storage capacity to a host entity 12. The PSDs 16 (and therefore the subsystem 14) are coupled to the host entity 12 through Parallel SCSI interconnects. Please note here that in this configuration, the host entity 12 is configured to issue IO requests in accordance with the specification of the Parallel SCSI interface standard and, therefore, is capable of accessing the PSDs 16 of the subsystem 14.
Similarly, in FIG. 2 a computer system 20 contains a JBOD subsystem 24 having a plurality of Fibre FC-AL PSDs 26 for providing storage capacity to a host entity 22. The PSDs 26 (and therefore the subsystem 24) are redundantly coupled to the host entity 22 through a pair of Fibre FC-AL interconnects. Please note here that, in this configuration, the host entity 22 is configured to issue IO requests in accordance with the specification of the Fibre FC-AL interface standard and, therefore, is capable of accessing the PSDs 26 of the subsystem 24. In addition, in this configuration, the redundant connection of the PSDs 26 through a pair of Fibre interconnects helps improve the accessibility of the PSDs 26. That is, under the situation that one of the pair of the Fibre interconnects breaks (due to, for example, cable failure or interface circuitry failure), the other Fibre interconnect can still provide the connection needed.
For the JBOD subsystems 14 and 24 in FIG. 1 and FIG. 2, because connection from the host entities 12, 22 to the PSDs is made using multiple device IO device interconnects, i.e., device IO interconnects that support multiple PSDs being coupled to the host entity through a single interconnect, without the need for any extra conversion circuitry. However, because SCSI/Fibre PSDs, especially HDDs, are expensive relative to their lower-end cousins, ATA PSDs, which do not incorporate an IO device interconnect that permits multiple devices to be connected on a single interconnect, the total cost of the JBOD subsystems 14, 24 based on this prior art technology is relatively high.